Traditional dose rates used in external beam radiotherapy are related to dose rates of available equipment and to the economic factors of technician time and machine time. Recent developments suggest that lower dose rates might be more effective. The objective of the proposed research is to determine the radiation dose-rate dependence of tumor cure in single-dose and fractionated radiotherapy of several mouse tumor types with three qualities of radiation. Results in the form of TCD50's (tumor cure dose for 50 percent of animals) for various dose rates, radiation qualities, fractionation schemes and tumor types will be analyzed in comparison with the dose-rate dependence of radiation damage to several normal tissues for which dose-rate data are available. Such comparison will help to determine conditions for an optimum dose-rate effect on tumor cure relative to normal tissue damage.